int led[] = {13, 12, 11, 10, 9, 8, 7};

void setup() {
  for(int i = 0; i < 7;i++){
    pinMode(led[i], OUTPUT);
  }
}
void loop() {
  for(int i = 0; i <= 7; i+=2){
    digitalWrite(led[i], 1);
    delay(500);
    digitalWrite(led[i], 0);
    delay(500);
  }
}


    // digitalWrite(led[i], 1);
    // digitalWrite(led[6-i], 1);
    // delay(500);
    // digitalWrite(led[i], 0);
    // digitalWrite(led[6-i], 0);
    // delay(500);